Heart rate variability as stress indicator

Using a portable ECG data logger, heart rate variability (HRV) in horses wa s measured under controlled(resting-), mental stress- and physical stress c onditions. Time domain-, Frequency domain- and non-linear parameters were u sed to describe the variations in the HRV. Simple linear parameters like SD ANN and RMSSD can demonstrate that there are differences in HRV between res ting conditions, mental- and physical-stress conditions but do not give an explanation about the underlying mechanisms. Fourier analysis with calculat ion of low and high frequency bands gives further information: physical str ess is characterised by an increase in sympathetic tone and a decrease in p arasympathetic tone - a fact which is commonly accepted. No differences cou ld be found between resting conditions and mental stress. Summarising the r esults of non-linear recurrence plot variables, it seems, that during diffe rent physiological states the organism shifts between various control strat egies. Under resting conditions a kind of deterministic-chaotic control is preferred which changes to more strict periodic control under physical stre ss. Mental stress seems to be characterised by a more random influenced con trol which means more or less loss of control over the heart rate variabili ty.